NAME

disklabel - disk pack label

SYNOPSIS

#include <sys/disklabel.h>

DESCRIPTION

Each disk or disk pack on a system may contain a disk label which pro-
vides detailed information about the geometry of the disk and the parti-
tions into which the disk is divided. It should be initialized when the
disk is formatted, and may be changed later with the disklabel(8) pro-
gram. This information is used by the system disk driver and by the
bootstrap program to determine how to program the drive and where to find
the filesystems on the disk partitions. Additional information is used by
the filesystem in order to use the disk most efficiently and to locate
important filesystem information. The description of each partition con-
tains an identifier for the partition type (standard filesystem, swap
area, etc.). The filesystem updates the in-core copy of the label if it
contains incomplete information about the filesystem.
The label is located in sector number LABELSECTOR of the drive, usually
sector 0 where it may be found without any information about the disk
geometry. It is at an offset LABELOFFSET from the beginning of the sec-
tor, to allow room for the initial bootstrap. The disk sector containing
the label is normally made read-only so that it is not accidentally
overwritten by pack-to-pack copies or swap operations; the DIOCWLABEL
ioctl(2), which is done as needed by the disklabel(8) program, allows
modification of the label sector.
A copy of the in-core label for a disk can be obtained with the
DIOCGDINFO ioctl; this works with a file descriptor for a block or char-
acter ("raw") device for any partition of the disk. The in-core copy of
the label is set by the DIOCSDINFO ioctl. The offset of a partition can-
not generally be changed while it is open, nor can it be made smaller
while it is open. One exception is that any change is allowed if no label
was found on the disk, and the driver was able to construct only a skele-
tal label without partition information. The DIOCWDINFO ioctl operation
sets the in-core label and then updates the on-disk label; there must be
an existing label on the disk for this operation to succeed. Thus, the
initial label for a disk or disk pack must be installed by writing to the
raw disk. The DIOCGPDINFO ioctl operation gets the default label for a
disk. This simulates the case where there is no physical label on the
disk itself and can be used to see the label the kernel would construct
in that case. The DIOCRLDINFO ioctl operation causes the kernel to update
its copy of the label based on the physical label on the disk. It can be
used when the on-disk version of the label was changed directly or, if
there is no physical label, to update the kernel's skeletal label if some
variable affecting label generation has changed (e.g. the fdisk partition
table). All of these operations are normally done using disklabel(8).
Note that when a disk has no real BSD disklabel the kernel creates a de-
fault label so that the disk can be used. This default label will include
other partitions found on the disk if they are supported on your archi-
tecture. For example, on systems that support fdisk(8) partitions the de-
fault label will also include DOS and Linux partitions. However, these
entries are not dynamic, they are fixed at the time disklabel(8) is run.
That means that subsequent changes that affect non-OpenBSD partitions
will not be present in the default label, though you may update them by
hand. To see the default label, run disklabel(8) with the -d flag. You
can then run disklabel(8) with the -e flag and paste any entries you want
from the default label into the real one.

SEE ALSO

CAVEATS

disklabel only supports up to a maximum of 15 partitions, 'a' through
'p', excluding 'c'. The 'c' partition is reserved for the entire physical
disk. By convention, the 'a' partition of the boot disk is the root par-
tition, and the 'b' partition of the boot disk is the swap partition, but
all other letters can be used in any order for any other partitions as
desired.
MirOS BSD #10-current August 6, 2001 1